The prime minister, along with the other ministers in cabinet, is appointed by the governor general on behalf of the monarch.[6] However, by the conventions of responsible government, designed to maintain administrative stability, the viceroy will call to form a government the individual most likely to receive the support, or confidence, of a majority of the directly-elected members of the House of Commons;[7] as a practical matter, this is often the leader of a party whose members form a majority, or a very large plurality, of Members of Parliament (MPs).[8] There are no age or citizenship restrictions on the position of prime minister itself.

Prime ministers who are not Members of Parliament upon their appointment (or who lose their seats while in office) have since been expected to seek election to the commons as soon as possible. For example, William Lyon Mackenzie King, after losing his seat in the 1925 federal election (that his party won), briefly "governed from the hallway" before winning a by-election a few weeks later. Similarly, John Turner replaced Pierre Trudeau as leader of the Liberal Party in 1984 and subsequently was appointed prime minister while not holding a seat in the House of Commons; Turner won a riding in the next election but the Liberal Party was swept from power. Turner was the last sitting prime minister to not hold a commons seat.

Should a sitting prime minister today lose his or her seat in the legislature, or should a new prime minister be appointed without holding a seat, the typical process that follows is that a junior member in the governing political party will resign to allow the prime minister to run in the resulting by-election.[10] A safe seat is usually chosen; while the Liberal and Conservative parties traditionally observed a convention of not running a candidate against another party's new leader in the by-election, the New Democrats and smaller political parties typically do not follow the same convention. However, if the governing party selects a new leader shortly before an election is due, and that new leader is not a member of the legislature, he or she will normally await the upcoming election before running for a seat in parliament.

The Canadian prime minister serves at Her Majesty's pleasure, meaning the post does not have a fixed term. Once appointed and sworn in by the governor general, the prime minister remains in office until he or she resigns, is dismissed, or dies.[13] The lifespan of parliament is limited by the constitution to five years and, though the governor general may still, on the advice of the prime minister, dissolve parliament and issue the writs of election prior to the date mandated by the Canada Elections Act; the King–Byng Affair was the only time since Confederation that the viceroy deemed it necessary to refuse his prime minister's request for a general vote.

Following parliamentary dissolution, the prime minister must run in the resulting general election if he or she wishes to maintain a seat in the House of Commons. Should the prime minister's party subsequently win a majority of seats in the House of Commons, it is unnecessary to re-appoint the prime minister or again swear him or her into office.[13] If, however, an opposition party wins a majority of seats, the prime minister may resign or be dismissed by the governor general. Should the prime minister's party achieve a minority while an opposition party wins a plurality (i.e., more seats than any other party but less than a majority), the prime minister can attempt to maintain the confidence of the House by forming a coalition with other minority parties. This option was last entertained in 1925.

Because the prime minister is, in practice, the most politically powerful member of the Canadian government, he or she is sometimes erroneously referred to as Canada's head of state,[n 2] when, in fact, that post is held by the Canadian monarch, represented by the governor general.[14] The prime minister is, instead, the head of government and is responsible for advising the Crown on how to exercise the Royal Prerogative and its executive powers,[2] which are governed by the constitution and its conventions. However, the function of the prime minister has evolved with increasing power. Today, as per the doctrines of constitutional monarchy, the advice given by the prime minister is ordinarily binding, meaning the prime minister effectively carries out those duties ascribed to the sovereign and/or governor general, leaving the latter to act in predominantly ceremonial fashions.[15] As such, the prime minister, supported by the Office of the Prime Minister (PMO), controls the appointments of many key figures in Canada's system of governance, including the governor general, the Cabinet, justices of the Supreme Court, senators, heads of crown corporations, ambassadors to foreign countries, the provincial lieutenant governors, and approximately 3,100 other positions. Further, the prime minister plays a prominent role in the legislative process—with the majority of bills put before parliament originating in the Cabinet—and the leadership of the Canadian Armed Forces.

Pierre Trudeau is credited with, throughout his tenure as prime minister between 1968 and 1984, consolidating power in the PMO,[16] which is itself filled by political and administrative staff selected at the prime minister's discretion and unaccountable to parliament. At the end of the 20th century and into the 21st, analysts—such as Jeffrey Simpson,[17]Donald Savoie, Andrew Coyne,[18] and John Gomery—argued that both parliament and the Cabinet had become eclipsed by prime ministerial power;[n 3][19] Savoie wrote: "The Canadian prime minister has little in the way of institutional check, at least inside government, to inhibit his ability to have his way."[20] Indeed, the position has been described as undergoing a "presidentialisation",[16][21] to the point that its incumbents publicly outshine the actual head of state;[22][23] former governor general Adrienne Clarkson alluded to what she saw as "an unspoken rivalry" that had developed between the prime minister and the Crown.[24] It has been theorized that such is the case in Canada as its parliament is less influential on the executive than in other countries with Westminster parliamentary systems; particularly, Canada has fewer MPs, a higher turnover rate of MPs after each election, and an Americanised system for selecting political party leaders, leaving them accountable to the party membership rather than caucus, as is the case in the United Kingdom.[25]

There do exist checks on the prime minister's power: the commons may revoke its confidence in an incumbent prime minister and Cabinet or caucus revolts can quickly bring down a sitting premier and even mere threats of such action can persuade and/or compel a prime minister to resign his post, as happened with Jean Chrétien. The Reform Act, 2014,[26] codifies the process by which a caucus may trigger a party leadership review and, if necessary, chose an interim leader, thereby making a prime minister more accountable to the MPs in his or her party. Caucuses may choose to follow these rules, though the decision would be made by recorded vote, thereby subjecting the party's choice to public scrutiny.[27]

The Senate may delay or impede legislation put forward by the Cabinet, such as when Brian Mulroney's bill creating the Goods and Services Tax (GST) came before the upper chamber and, given Canada's federal nature, the jurisdiction of the federal government is limited to areas prescribed by the constitution. Further, as executive power is constitutionally vested in the monarch, meaning the Royal Prerogative belongs to the Crown and not to any of its ministers,[28][29][30] the sovereign's supremacy over the prime minister in the constitutional order is thus seen as a "rebuff to the pretensions of the elected: As it has been said, when the Prime Minister bows before the Queen, he bows before us [the Canadian people]."[31][32] Either the sovereign or his or her viceroy may therefore oppose the prime minister's will in extreme, crisis situations.[n 4] Near the end of her time as governor general, Adrienne Clarkson stated: "My constitutional role has lain in what are called 'reserve powers': making sure that there is a prime minister and a government in place, and exercising the right 'to encourage, to advise, and to warn'[...] Without really revealing any secrets, I can tell you that I have done all three."[33]

Should a sitting or former prime minister die, he or she is accorded a state funeral, wherein their casket lies in state in the Centre Block of Parliament Hill.[37] Only Bowell and the Viscount Bennett were given private funerals, Bennett also being the only former Prime Minister of Canada to die and be buried outside the country and Bowell the only whose funeral was not attended by politicians. John Thompson also died outside Canada, at Windsor Castle, where Queen Victoria permitted his lying-in-state before his body was returned to Canada for a state funeral in Halifax.[38]

The mark of the Prime Ministership of Canada, applied to the arms of prime ministers who pursue them

In earlier years, it was traditional for the monarch to bestow a knighthood on newly appointed Canadian prime ministers. Accordingly, several carried the prefix Sir before their name; of the first eight premiers of Canada, only Alexander Mackenzie refused the honour of a knighthood from Queen Victoria. Following the 1919 Nickle Resolution, however, it was against non-binding policy for the sovereign to grant such honorific titles to Canadians; the last prime minister to be knighted was Sir Robert Borden, who was premier at the time the Nickle Resolution was debated in the House of Commons. Still, Bennett was in 1941, six years after he stepped down as prime minister, elevated to the peerage by King George VI as Viscount Bennett, of Mickleham in the County of Surrey and of Calgary and Hopewell in the Dominion of Canada.[39][40]

Kim Campbell, the 19th Prime Minister of Canada (1993) and only female and British Columbia-born individual to hold the office

Canada continues the Westminster tradition of using the title Prime Minister when one is speaking to the federal head of government directly; this is in contrast to the United States protocol of addressing the federal head of government as mister (as in, Mister President); the Department of Canadian Heritage advises that it is incorrect to use the term Mr Prime Minister.[46] The written form of address for the prime minister should use his or her full parliamentary title: The Right Honourable [name], [post-nominal letters], Prime Minister of Canada. However, while in the House of Commons during Question Period, other members of parliament may address the prime minister as The Right Honourable, Member for [prime minister's riding] or simply The Right Honourable Prime Minister.[47] Former prime ministers retain the prefix The Right Honourable for the remainder of their lives; should they remain sitting MPs, they may be referred as The Right Honourable Member for [member's riding] or by their portfolio title (if appointed to one), as in The Right Honourable Minister of National Defence.

In the decades following Confederation, it was common practice to refer to the prime minister as Premier of Canada,[48][49][50] a custom that continued until the First World War, around the time of Robert Borden's premiership.[51][52][53] While contemporary sources will still speak of early prime ministers of Canada as premier,[54][55][56] the modern practice is such that the federal head of government is known almost exclusively as the prime minister, while the provincial heads of government are termed premiers (save for within Quebec and New Brunswick, where the premiers are addressed in French as Premier ministre du [province], literally translated as Prime Minister of [province]).

After exiting office, former prime ministers of Canada have engaged in various pursuits. Some remained in politics: Bowell continued to serve as a senator, Stephen Harper returned to the House of Commons as a backbench Member of Parliament, and Bennett moved to the United Kingdom after being elevated to the House of Lords.[57] A number led Her Majesty's Loyal Opposition in the Canadian parliament: John A. Macdonald, Arthur Meighen, Mackenzie King,[58] and Pierre Trudeau, all before being re-appointed as prime minister (Mackenzie King twice); Alexander Mackenzie and John Diefenbaker, both prior to sitting as regular Members of Parliament until their deaths;[59]Wilfrid Laurier dying while still in the post;[60] and Charles Tupper,[61]Louis St. Laurent,[62] and John Turner, each before they returned to private business. Meighen was also appointed to the Senate following his second period as prime minister, but resigned his seat to seek re-election and moved to private enterprise after failing to win a riding.[63] Following Meighen into civilian life were: Robert Borden, who served as Chancellor of Queen's and McGill Universities, as well as working in the financial sector; Lester B. Pearson, who acted as Chancellor of Carleton University;[64] Joe Clark and Kim Campbell, who became university professors, Clark also consultant and Campbell working in international diplomacy and as the director of private companies and chairperson of interest groups; while Pierre Trudeau and Jean Chrétien returned to legal practice.[65] Former prime ministers also commonly penned autobiographies—Tupper,[61] for example—or published their memoirs—such as Diefenbaker and Paul Martin.[59]